1. Field of the Invention
The present invention relates to a variable level converter, and more particularly relates to a level adjusting circuit which is preferably utilized in an AGC (Automatic Gain Control) circuit having a negative feedback loop therein.
2. The Prior Art
The variable level converter is one of the most important circuit units in various kinds of electronically controlled apparatuses. This variable level converter is generally comprised of a conventional variable impedance element, such as an FET (Field Effect Transistor), and a diode or a transistor. The variable level converter is usually used for, firstly, converting the level of a signal to the other desired signal level in dependence upon an external level specifying signal, and secondly for creating the AGC circuit together with an amplifier. In the AGC circuit an output level of the amplifier is supplied, as a level specifying signal, to the variable level converter, by way of the feedback loop, so as to keep the output level at a desired constant.
Such a variable level converter is widely employed in various kinds of electronically controlled systems, such as a microwave transmitting system, a microwave receiving system or a satellite communication system.
In the prior art, the variable level converter which is comprised of both one variable impedance element and the other circuit element, for example the so-called inverted L-type variable attenuator, has widely been known in the world. This variable level converter can convert the level of a signal to the other desired signal level; also the variable level converter can perform the AGC operation, by utilizing the so-called non-linear characteristics which is created by the variable impedance element. However, the variable level converter of the prior art has a defect. The defect is that it is difficult to obtain a wide range of level conversion. This is because, when the level is converted in a wide range, a deleterious non-linear distortion, especially a second-order non-linear distortion, is produced from the variable impedance element. In order to maintain a high quality level conversion, such a second-order non-linear distortion must be eliminated from the variable level converter. Further, it is required, for the variable impedance element, to have a wide range of impedance value variation so as to obtain a wide range of the level conversion. However, the more the impedance value varies in a wide range, the more the noise may increase, due to the second-order non-linear distortion.